L-band Linac Research Articles

First demonstration of coherent resonant backward diffraction radiation for a quasi-monochromatic terahertz-light source

We proposed coherent resonant backward diffraction radiation (CRBDR), which generates wavelength-tunable quasi-monochromatic lights using a compact diffractor assembly in an accelerator facility of high-energy electron beams, as a unique intense terahertz (THz) light source. Superimposing the coherent backward diffracted radiation emitted by periodically arranged hollow diffractors, it is possible to amplify the frequency components satisfying a resonant condition, and make the radiation monochromatic. We demonstrated the CRBDR using the L-band linac at the Institute for Integrated Radiation and Nuclear Science at Kyoto University. It was observed that the coherent backward diffraction radiation was amplified more than three times at a frequency which was the fundamental resonant frequency in the CRBDR theory. Moreover, the number of diffractors at the saturation of the radiation power was consistent with the number estimated from the electron distribution in a bunch. The experimental results show that the CRBDR is useful as a quasi-monochromatic light source in the THz band.

An SLC-Type e<sup>+</sup/> e<sup>−</sup/>/γγ Facility at a Future Circular Collider

It is proposed to place the arcs of an SLC-type facility inside the tunnel of a Future Circular Collider (FCC). Accelerated by a linear accelerator (linac), electron and positron beams would traverse the bending arcs in opposite directions and collide at centre-of-mass energies considerably exceeding those attainable at circular $e^{+}e^{-}$ colliders. The proposed SLC-type facility would have the same luminosity as a conventional two-linac $e^{+}e^{-}$ collider. Using an optical free-electron laser, the facility could be converted into a $\gamma\gamma$ collider. A superconducting L-band linac at the proposed facility may form a part of the injector chain for a 100-TeV proton collider in the FCC tunnel. The whole accelerator complex would serve as a source of $e^{+}e^{-}$, $\gamma\gamma$, $pp$ and $ep$ interactions. The L-band linac could also be used to produce high-intensity neutrino, kaon and muon beams for fixed-target experiments, as well as X-ray free-electron laser (XFEL) photons for applications in material science and medicine.

Longitudinal bunch shaping of picosecond high-charge MeV electron beams

With ever increasing demands for intensities in modern accelerators, the understanding of space-charge effects becomes crucial. Herein are presented measurements of optically shaped picosecond-long electron beams in a superconducting L-band linac over a wide range of charges, from 0.2 nC to 3.4 nC. At low charges, the shape of the electron beam is preserved, while at higher charge densities, modulations on the beam convert to energy modulations. Energy profile measurements using a spectrometer and time profile measurements using a streak camera reveal the dynamics of longitudinal space-charge on MeV-scale electron beams.

Grid pulser for an electron gun with a thermionic cathode for the high-power operation of a terahertz free-electron laser

A grid pulser for a thermionic-cathode electron gun for an L-band electron linac with an RF frequency of 1.3GHz was developed in an effort to increase the power of a terahertz (THz) free-electron laser (FEL) based on the linac. The grid pulser can generate a train of electron pulses with a 5-ns duration at intervals of 36.9ns or at a repetition frequency of 27MHz, which is the 48th sub-harmonic of the RF frequency, and with a peak current of up to 2.4A or higher, which is four times higher than the current that can be obtained with the conventional grid pulser. In combination with the sub-harmonic buncher (SHB) system, whose fundamental frequency is 108MHz (the 12th sub-harmonic of the RF frequency), an electron beam that comprises electron bunches separated by 36.9-ns intervals for a period of 8μs can be successfully accelerated using the linac to an energy of 15MeV under the condition that the average beam current or, correspondingly, the beam loading in the acceleration tube of the linac is the same as that obtained using the conventional grid pulser. The time-resolved energy spectrum indicates that the energy is constant over the latter 6μs and that the energy spread is 1.2% (FWHM); this performance satisfies the requirements for the FEL. The bunch charge measured at the end of the FEL beamline is 4nC, which is four times higher than that obtained using the conventional grid pulser.

Verification of terahertz-wave spectrophotometry by Compton backscattering of coherent synchrotron radiation

Abstract We developed a continuous-spectrum light beam from Compton backscattering by using coherent synchrotron radiation in an L-band linac at the Kyoto University Research Reactor Institute. The ratio of Compton backscattered photons to background photons when coherent synchrotron radiation was used was three times larger than when coherent transition radiation was used. The transmission spectrum of a polystyrene film in the terahertz-wave region was evaluated by measuring the spectrum of the Compton backscattered photons and it roughly agreed with that measured by a Martin–Puplett-type interferometer. The spectrophotometry using Compton backscattering shows promise as a new tool for investigations in terahertz-wave science.

The high-power operation of a terahertz free-electron laser based on a normal conducting RF linac using beam conditioning

The feed-forward control of the amplitude and phase of a high-power RF pulse based on the overdrive technique is applied to an L-band electron linac to ensure that the energy of the electron beam is uniform and constant enough for the recommissioning of a terahertz (THz) free-electron laser (FEL) at the Institute of Scientific and Industrial Research (ISIR), Osaka University. The quality of the electron beam is evaluated using the time-resolved measurement of its energy spectrum; the energy is constant within 0.1% during the electron macropulse for 15MeV energy except for the transient period in the leading edge. A high-power, stable FEL operation that reaches saturation is obtained using beam conditioning. The characteristics and performance of the FEL for THz radiation are measured at the end of an evacuated transport line. The frequency ranges from 2 to 12THz, from 25 to 150μm in wavelength, and the maximum energy in the FEL macropulse with a ~3μs duration is 3.7mJ at 67μm; using these values, the peak power in the macropulse is calculated to be 1.2kW, while that in the micropulse with a 20ps duration is calculated to be 0.5MW. These values are comparable to those in the THz range of other FELs that are based on normal conducting RF linacs.

?Compact Submillimeter FEL Project

Recently, interest in scientific research using submillimeter electromagnetic waves has been increasing rapidly, and with it, available submillimeter sources are either too weak or too big and expensive. Therefore, a compact submillimeter FEL project is being developed now at Budker Institute of Nuclear Physics. The machine is based on a dedicated 10-MeV L-band linac. All aspects of the feasibility study of the project are discussed. The expected parameters of the machine are listed.

Suppression of Initial Energy Spreads in Electron Radio Frequency Linacs for Intense Irradiation Applications

RF linacs for high-power beams are operated under the fully beam-loaded condition for power efficiency. Under this condition, initial energy spreads are generated by the transient beam-loading effect. Irradiation sources require a beam energy of less than 10 MeV for radiation safety. In order to maximize the beam power and maintain the beam energy at a safe level, we need to suppress the initial energy spreads. In an L-band traveling-wave linac for irradiation applications, the initial high-energy electrons are suppressed by beam current modulation with RF power modulation. In this study, the temporal profile of beam energy is calculated by a numerical method and verified by measurement. As a result, the average beam energy and the corresponding beam power are improved by nearly 60% compared with the case without any modulations. The effect of beam current modulation on bunching and acceleration is also discussed in this paper.

Compact THz Fel Project

Interest in scientific research using terahertz electromagnetic waves is rapidly increasing recently. At the same time sources of radiation of this range either are very low-power, or too bulky and expensive. A compact FEL being developed recently at the Budker Institute of Nuclear Physics is exactly a relatively inexpensive and powerful machine. The FEL is based on a dedicated L-band linac. All the aspects of the feasibility study of the project are discussed. The expected parameters of the machine are listed

Development of the compact electron spectrometer for the FIREX-I Project in Gekko XII

The high energetic electron measurement is one of the most important issues to research the ignition mechanism in the Fast Ignition Realization EXperiment Project. It is also important for the energy spectra with angular distribution because the electron spread is different by the target design. Therefore we have been developed the compact Electron SpectroMeter so as to be installed on different angular potions. We have performed the calibration using L-band LINAC in the Institute of Scientific and Industrial Research, Osaka University. The analyzer has been tested to measure energetic electrons from the aluminum and gold plain targets irradiated by LFEX laser (maximum energy of 10 kJ) up to 800 J. The electron measurement has been performed at the integrated experiments using CD-shell with Au cone. The non-Maxwellian spectrum can be observed when the effective core heating by the electron is occurred.

Short-Range Longitudinal Wake Field and Its Effect on the Energy Spectrum in a Linear Accelerator

We have studied the effects of the wake field on the energy spectrum of a high-intensity single-bunch electron beam of 28 nC charge accelerated in an L-band accelerating structure of 1.3 GHz frequency both by experiment and by calculation. Energy spectra measured as a function of the accelerating phase show significant changes, including appearance of multiple peaks owing to the short-range longitudinal wake field. The empirical wake function is derived for the specific accelerating structure using the simulation code, ABCI. Energy spectra calculated with the wake function agree quite well with the energy spectra measured over the wide range of the accelerating phase. The effects of the wake field on the energy spectrum of the high-intensity single-bunch beam accelerated with the L-band linac at Institute of Scientific and Industrial Research (ISIR), Osaka University, are quantitatively explained in terms of the wake field theory with the empirical wake function.

Beam breakup in one-turn and two-turn energy recovery linacs

The capital and operating costs of an energy recovery linac (ERL) may be reduced by multi-turn operation, which uses more than one pass of acceleration and deceleration. However, the beam current limit due to multipass beam breakup (BBU) may be decreased. As an example, we study a 1-GeV ERL with average current of 100 mA, which is a possible source of bright synchrotron radiation. A one-turn design with a single superconducting L-band linac is compared to a two-turn single-linac design. In simulations, the BBU threshold currents of the two-turn design are about one-half as large as the one-turn thresholds. If the linac’s strongest higher-order modes are damped to obtain quality factors of a few thousand, 100-mA currents are possible in both designs. Thus, a two-turn ERL may be a feasible path to reduce the linac cost and machine size.

Energy monitoring device for 1.5–2.4 MeV electron beams

An easy-to-use and robust energy monitoring device has been developed for reliable detection of day-to-day small variations in the electron beam energy, a critical parameter for quality control and quality assurance in industrial radiation processing. It has potential for using on-line, thus providing real-time information. Its working principle is based on the measurement of currents, or charges, collected by two aluminium absorbers of specific thicknesses (dependent on the beam energy), insulated from each other and positioned within a faraday cup-style aluminium cage connected to the ground. The device has been extensively tested in the energy range of 4–12 MeV under standard laboratory conditions at Institute of Isotopes and CNR-ISOF using different types of electron accelerators; namely, a TESLA LPR-4 LINAC (3–6 MeV) and a L-band Vickers LINAC (7–12 MeV), respectively. This device has been also tested in high power electron beam radiation processing facilities, one equipped with a 7-MeV LUE-8 linear accelerator used for crosslinking of cables and medical device sterilization, and the other equipped with a 10 MeV Rhodotron TT100 recirculating accelerator used for in-house sterilization of medical devices. In the present work, we have extended the application of this method to still lower energy region, i.e. from 1.5 to 2.4 MeV. Also, we show that such a device is capable of detecting deviation in the beam energy as small as 40 keV.

Calibration of Compact Electron Spectrometer for the FIREX-I Project in Gekko XII

We have developed a compact electron spectrometer (ESM) to investigate ignition mechanism in the Fast Ignition Realization Experiment project. Hot electrons, produced by the irradiation of a gold target by using a compressed chirped pulse laser, are used for auxiliary heating of the imploded core. An imaging plate is used as the electron detector for medical purposes. However, the relationship between the beam intensity and the detector signal is not clear. The ESM should be calibrated because it is highly compact and has a complex magnetic field for bending caused by installation in a limited space. We have performed the calibration by using an L-band LINAC at the Institute of Scientific and Industrial Research, Osaka University, in order to obtain an accurate electron spectrum. The calibration used a single electron pulse at two different energies, 9.5 MeV and 27.1 MeV, with 0.1-10 pC. The energy spreads are 0.2 MeV at 9.5 MeV and 0.3 MeV at 27.1 MeV. The analyzer has been tested to measure energetic electrons from plain and integrated targets irradiated by the Laser or Fast Ignition Experiment (maximum energy of 10 kJ) up to 800 J.

Development of FEL and SASE in the far-infrared region at ISIR, Osaka University

Free electron laser (FEL) and self-amplified spontaneous emission (SASE) are being developed in the far-infrared region using the L-band electron linac at the Institute of Scientific and Industrial Research (ISIR), Osaka University. The L-band linac was recently remodeled extensively not only for higher operational stability and reproducibility but also for high power operation of FEL. After commissioning of the linac, we first began SASE experiment with a newly-developed strong-focusing wiggler. Recently we began FEL experiment and obtained lasing with the high peak power at 70μm again after a long break.

Calibration of imaging plate for high energy electron spectrometer

A high energy electron spectrometer has been designed and tested using imaging plate (IP). The measurable energy range extends from 1to100MeV or even higher. The IP response in this energy range is calibrated using electrons from L-band and S-band LINAC accelerator at energies 11.5, 30, and 100MeV. The calibration has been extended to 0.2MeV using an existing data and Monte Carlo simulation Electron Gamma Shower code. The calibration results cover the energy from 0.2to100MeV and show almost a constant sensitivity for electrons over 1MeV energy. The temperature fading of the IP shows a 40% reduction after 80min of the data taken at 22.5°C. Since the fading is not significant after this time we set the waiting time to be 80min. The oblique incidence effect has been studied to show that there is a 1∕cosθ relation when the incidence angle is θ.

Temporal structure of resonator output in a millimeter-wave prebunched FEL

A short-bunched beam of electrons of an L-band linac of 38 MeV is guided to a cylindrical-closed resonator. Wavepackets of coherent transition radiation (CTR) generated from bunches passing through the mirrors are coherently summed up in the resonator. The amplification of the output of the resonator is confirmed in comparison with spontaneous CTR. The temporal structure of the output, observed with a hot-electron bolometer having the response time of 0.35 μs , shows oscillating characteristic of about 1 μs in time scale. The result strongly suggests the inter-bunch distance varies rapidly over the duration of a macropulse, and it consequently results in degradation of the coherent summing up in the resonator.

Temporal structure of resonator output in a millimeter-wave prebunched FEL

A short-bunched beam of electrons of an L-band linac of 38MeV is guided to a cylindrical-closed resonator. Wavepackets of coherent transition radiation (CTR) generated from bunches passing through the mirrors are coherently summed up in the resonator. The amplification of the output of the resonator is confirmed in comparison with spontaneous CTR. The temporal structure of the output, observed with a hot-electron bolometer having the response time of 0.35μs, shows oscillating characteristic of about 1μs in time scale. The result strongly suggests the inter-bunch distance varies rapidly over the duration of a macropulse, and it consequently results in degradation of the coherent summing up in the resonator.

Higher harmonic generation observed in SASE in the far-infrared region

The self-amplified spontaneous emission (SASE) has been generated using the single bunch electron beam of 11.3 MeV energy with a peak current of ∼0.5 kA accelerated with an L-band linac and a 1.92 m long, 32 period wiggler, and the wavelength spectrum of SASE has been measured in the wide wavelength region from 50 to 200 μm using a grating monochromator. We have observed the second harmonic peak at 90 μm and the third harmonic peak at 60 μm in the spectrum as well as the fundamental peak of SASE at 180 μm. Spectral widths of the fundamental, the second and the third harmonics are measured to be 2.9%, 3.6%, 5.3%, respectively. The FEL parameter is 0.032, which is estimated from the spectral width of the fundamental peak. Absolute energies per optical pulse of the fundamental, the second and the third harmonics have been derived to be 2.75 nJ, 73 pJ, 182 pJ, respectively, by calibrating the measurement system with a blackbody radiator. The second and the third harmonic peaks are concluded to be produced by nonlinear harmonic generation of SASE.

Wavelength spectrum of self-amplified spontaneous emission in the far-infrared region

We are conducting experiments to generate self-amplified spontaneous emission (SASE) in the far-infrared region and to measure its characteristics, using a single bunch electron beam accelerated with the L-band linac at ISIR, Osaka University. We have measured the wavelength spectra of SASE in the wavelength region between 80 and 180 μm using a grating monochromator and a Ge : Ga detector. The measured spectral widths of the fundamental peak are slightly larger than those predicted by the one-dimensional model. We have also observed the second harmonic peak of SASE.